Decaffeination of aqueous extracts

ABSTRACT

Green coffee is decaffeinated in a continuous economical process without organic solvent. Caffeine is extracted from a solution of green coffee solubles with supercritical carbon dioxide in a novel extractor under specified conditions.

This application is a continuation-in-part of U.S. patent applicationSer. No. 0,60,956 filed June 15, 1987, now abandoned, which is acontinuation of application No. 821,868 filed Jan. 23, 1986 which was acontinuation of application No. 479,263 filed Mar. 28, 1983, nowabandoned.

BACKGROUND OF THE INVENTION

The present invention relates generally to extraction processes and moreparticularly to a decaffeination of coffee by a continuous extractionprocess. A caffeine-laden aqueous extract of green coffee is preparedfrom which caffeine is extracted by the use of carbon dioxide as asolvent. The invention lies in the use of specific equipment underclosely controlled conditions of pressure and temperature to providehigh yields and economical costs of production.

In recent years, consumer demand for decaffeinated coffee has beenincreasing. About 20% of the coffee sold in the United States isdecaffeinated. Most of this coffee is now decaffeinated in processeswhich employ synthetic organic solvents such as methylene chloride orethyl acetate, either by directly contacting the coffee beans with theextracting organic solvent or indirectly, by contact of coffee beanswith solvent-decaffeinated coffee extract.

There is public concern that decaffeinated coffee may containinadvertent traces of organic solvent which might be toxic orcarcinogenic. Furthermore, organic solvents remove other coffeeconstituents along with caffeine to the detriment of the coffee flavor.There has thus arisen a need for a process which does not use organicsolvent.

The use of pressurized gases as solvents has long been known. Forexample see M. Centnerszwer, Z. Physik. Chem. 46, 427 (1903); H.Buchner, Z. Physik. Chem. 54, 665 (1906); M. Centnerszwer, Z. Physik.Chem. 72, 431 (1910); D. B. Todd and J. C. Elgin, A.I. Ch. E. Journal 1,20 (1955).

Carbon dioxide, a normal constituent of roasted coffee, is a preferredsolvent for decaffeination. It is produced naturally during the roastingprocess, typically to the extent of 2 to 3% of coffee weight. It isnon-toxic and will selectively remove caffeine from coffee beans orcoffee extract. It may be used for decaffeination at elevated pressurealthough much more carbon dioxide is required than conventional organicsolvents. Consequently, for economic operation, highly efficientdecaffeination equipment is required.

U.S. Pat. No. 4,260,639 to Zosel discloses a slow process, not readilymade continuous, for the carbon dioxide decaffeination of coffee frombatches of moistened green coffee beans in a period of 5 to 30 hours. Inthat process each batch of green (unroasted) coffee beans is extractedin a high pressure vessel which must be charged and closed. The carbondioxide must then be admitted and pumped to a high pressure beforedecaffeination can be effected. After decaffeination the pressure mustbe released slowly so that the beans do not fracture by expansion of thecarbon dioxide inside the beans. The vessel is then opened to dischargethe decaffeinated coffee. The cost and time required for loading,caffeine extraction and unloading and the cost of re-pressurizing thecarbon dioxide after each batch make this process economicallyunattractive.

An indirect "water process" decaffeination method is described inBritish Patent No. 314,059 to Klapproth and U.S. Pat. No. 2,309,092 toBerry et al. However, they employ an organic solvent, generally achlorinated solvent, to decaffeinate an aqueous solution of green coffeesolubles. The aqueous solution is then used to extract caffeine fromgreen coffee beans. The caffeine-laden solution is recycled to thesolvent. If water were used instead of the aqueous coffee solution, mostof the coffee flavor precursors which develop flavor and aroma onsubsequent roasting would be lost with the water and a substantial partof the coffee weight would also be lost. Recycling of the solutionresults in the removal from the coffee of primarily only thosecomponents (principally caffeine) which are extracted by the organicsolvent from the solution. The "water process" has also been modified byothers to use solid adsorbents such as activated carbon instead of achlorinated organic solvent to remove caffeine from the solution. Thismethod has the disadvantage that some other coffee components areremoved as well as caffeine, reducing yield and weakening the coffeeflavor. See for example, European patent application 111,375 toMoolweer.

German patent application No. 2,638,383 to van der Stegen describes acontinuous process which substitutes carbon dioxide for the chlorinatedorganic solvent in the "water process". The van der Stegen suggestionhas the advantage that coffee beans can be extracted with a watersolution at low or atmospheric pressure and only this solution need behandled at elevated pressure in contact with carbon dioxide. The van derStegen idea has been open to public inspection since Mar. 3, 1977, yetthere has been no known commercial application of the process.

Extraction requires the transfer of a solute (caffeine) from one fluid(green coffee extract) to a second fluid (pressurized carbon dioxide).Close contact of the fluids followed by their effective separation isessential. Generally a series of such contacts and separations arrangedcountercurrently is required for efficient extraction. I have found thatthe green coffee extract has a strong tendency to form a foam whenintermixed with pressurized carbon dioxide. This foam makes caffeineextraction and separation of the carbon dioxide from the solution assuggested by van der Stegan impractical in equipment of conventionaldesign.

Packed extraction columns, such as described in U.S. Pat. No. 4,348,422to Zosel are unsuitable because there is considerable vertical (axial)mixing of the two fluids resulting in poor efficiency and increased costof operation. This is particularly severe in systems employingsupercritical gases such as carbon dioxide; its low viscosity (about onefifteenth that of water) increases eddy flow and thereby interferes withthe countercurrent contacting required for efficient extraction.

Mechanically assisted columns such as "rotating disc" columns or"Scheibel" columns are also subject to axial mixing. The mechanicalagitation also promotes formation of emulsion or foam which inhibitsseparation of the two fluids.

Sieve plate extraction columns are well known to effectively preventbackmixing. See C. J. King "Separation Processes" McGraw-Hill, New York1980, page 765. Attempts to decaffeinate green coffee extract withcarbon dioxide in a conventional sieve plate column were unsuccessful.Contact and almost complete separation of the two fluids betweensuccessive plates in the column are essential for proper operation.Conventional design does not achieve this. Supercritical carbon dioxidein passing upward through plate perforations and through a green coffeeextract forms a persistent foam which prevents operation of the process.

Prior to the instant invention, there has not been an economical andefficient countercurrent extraction process for the decaffeination ofcoffee in a continuous process employing supercritical carbon dioxideand no organic solvents.

SUMMARY OF THE INVENTION

An object of this invention is to provide a method for makingdecaffeinated coffee efficiently and economically without the loss ofcoffee flavor and aroma precursors and without introducing any organicsolvent or other additive.

It is an object of the present invention to provide an economical andefficient continuous process employing a sieve plate column for theextraction of caffeine by supercritical carbon dioxide from an aqueousextract of green coffee beans.

It is a further object of the present invention to enhance theefficiency of such processes by providing means to inhibit the floodingcaused by separation-inhibiting foam. A further object of the presentinvention is to provide in conjunction with novel high pressuredecaffeination apparatus a method for the operation of such apparatus toprovide the efficient and economical decaffeination of coffee.

A further object of the present invention is to provide novel sievecolumn decaffeination apparatus having optimized anti-foam mesh area andvoid percentages.

A further object of the present invention is to provide a novelperforated plate column decaffeination apparatus including a foamsuppressing mesh network used in conjunction with the discovered methodof operation.

A further object of the present invention is to provide, in conjunctionwith novel decaffeination apparatus, a method of operation at optimalpressure, temperature, extract concentration, extract flow rate andcarbon dioxide flow rate to render the process economical and efficient.

Still other and further objects of the present invention will beapparent from the detailed description which follows.

I have discovered that an unexpected combination of extraction equipmentdesign and operating conditions attains the foregoing objectives.

The apparatus and technique disclosed herein may be used indecaffeinating extracts of roasted or of gree coffee. The preferredapplication is to the decaffeination of green coffee extract to be usedto produce decaffeinated green coffee beans. Decaffeination of greencoffee extract avoids loss during decaffeination of delicate andfugitive coffee flavors and aromas developed subsequently by roasting.

Carbon dioxide is used at elevated pressure. It is preferred to otherknown decaffeination solvents because it is non-toxic and, undersuitable conditions, highly selective for caffeine. However, it has somedisadvantages. Although very selective for caffeine, carbon dioxide is arelatively poor solvent for caffeine. Efficient extraction requires arelatively high ratio of carbon dioxide to coffee solution ofapproximately 30 to 35 is required for 99% decaffeination. The extremelylow viscosity of supercritical carbon dioxide also affects extractordesign.

In accordance with preferred embodiment of this invention aliquid-liquid extraction column of unique design is employed. Above itscritical temperature (87.8° F.), carbon dioxide cannot exist as a liquidregardless of the pressure and is therefore called a supercriticalfluid. At the elevated pressure used in the process to be described, thefluid approaches the density of some liquids. The present inventionemploys equipment designed for continuous countercurrent contact of twoliquids. Such equipment is an improvement for the present purpose uponthe prior art.

The present invention avoids the limitations found in a packedextraction column or a mechanically assisted column. Those columns areincapable of handling high solvent ratios of 10 or more. See R. B.Akell, Chemical Engineering Progress 62 (9) 1966 page 50 and E. D.Oliver, "Diffusional Separation Processes" John Wiley, New York 1966page 363.

It has now been discovered that modifying the design of a sieve platecolumn permits practical and economic decaffeination within limitedranges of operating conditions. A porous material providing an extendedsolid surface is placed above the liquid surface so as to contact thefoam rising from the liquid. It was found that the surface acted to"break" the foam and permit good operation of the extraction column.Anti-foam compounds were unacceptable here because chemccal additivesare incompatible with a "natural" food product. Control of the foamsolely by physical means was accomplished.

Surprisingly, it was also discovered that this foam "breaking" bysurface contact operates satisfactorily only when the system pressure isgreater than about 330 bar. Performance is better at about 380 bar andstill better at about 420 bar. Tests made at pressures up to about 480bar did not indicate any further improvement in separation. Sinceequipment costs increase as system pressure is increased, operation atabout 420 bar is preferred for economy.

Gravity causes the separation of two fluids by virtue of the differencein their densities. One would expect that the greater the densitydifference, the more effective the separation. This was discovered notto be true with supercritical carbon dioxide and aqueous green coffeesolution within the range of pressure indicated above. Carbon dioxide isthe lower in density of the two. As system pressure is increased, carbondioxide density increases while the aqueous solution remains almostunchanged. Thus, the density difference decreases as system pressure isincreased. Therefore, the discovered improvement in separationefficiency with increased pressure could not be anticipated.

Furthermore, it is known that increased pressure increases the mutualsolubilities of carbon dioxide and water which reduces their interfacialtension. Foam coalescence is favored by increased interfacial tension.Again, the discovered effect of increased pressure is contrary toexpectation.

The preferred extraction temperature is about 80° to 85° C. Temperaturesfrom about 75° to 100° C. can be used. However, above about 85° C. thereis a slow deterioration and discoloration of the aqueous green coffeeextract. Temperatures below about 80° C. increase foam viscosity in theextractor although the effect of small temperature changes nn viscosityof unfoamed extract is extremely small. Increased foam viscosity caninterfere with extractor operation even under otherwise optimumconditions.

The concentration of the aqueous green coffee solution may be betweenabout 18% and about 30% dissolved solubles. At higher concentrations thefoam viscosity apparently increases excessively, again despite a verysmall change in viscosity of the unfoamed solution. Surprisingly, thetendency to foam is increased at lower solubles concentration. Operationis best in the range of 22 to 27% solubles, preferably about 26%.

The structure of the porous solid surface employed to break the foam isvery important. In general, the larger the surface area made available,the better the performance. How ever, practical extractor design islimited by two other factors the hydraulics of sieve plate extractorsand the commercial availability of suitable materials.

The hydraulic relationships of conventional sieve plate extractors arewell known. See, for example, R. E. Treybal "Mass Transfer Operations",McGraw-Hill, New York 1980, pages 532 to 535. The maintenance ofsuitable layers of solution on the plates is affected by flow rates,fluid densities, and the physical design and dimensions of plates,perforations, etc. The addition of surface area for foam breaking cangreatly affect the extractor hydraulics and cause the column to fail tooperate. It has been discovered that only a limited range of structuresis effective for foam breaking without interference with extractorfunctioning.

Absent the porous material discovered to break foam, the column fillswith foam and fails to function (floods) almost immediately on startup.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a sieve plate extraction columnequipped with porous foam-breaking material of this invention. The viewis a vertical section at a diameter of a cylindrical extraction column.

FIG. 2 is a schematic illustration of the system employing the sieveplate extraction column of FIG. 1 in a continuous process.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

The preferred embodiment of the sieve plate extraction column 1 of thepresent invention is shown in FIG. 1. The column is capable of safelysustaining the preferred operating pressure of 420 bar. Inlet port means2 allows aqueous green coffee extract containing caffeine to beintroduced under pressure near the top of the column. Extract outletport means 3 allows decaffeinated green coffee extract to leave thebottom of the column. Carbon dioxide input port means 4 allows carbondioxide containing very little or no caffeine to enter under pressurenear the bottom of the column. Carbon dioxide output port 5 allowscarbon dioxide laden with caffeine removed from the aqueous extract toleave at the top of the column. The perforated sieve plates are depictedat 6. For clarity only three plates are shown although the column maycontain any number of plates. The plates and other components of thecolumn are arranged so that green coffee solution 7 forms a continuousbody of liquid from inlet port 2 to its outlet port 3, constituting acontinuous phase. The liquid being more dense than carbon dioxide, thesystem is configured so that the continuous phase solution flowsdownward through the column but is constrained to flow generallyhorizontally across each plate and then downward to the next lower platethrough a channel called a downcomer 8. The liquid cannot fall throughthe perforations on the plates because the carbon dioxide is risingthrough all the perforations at sufficient velocity. The plates 6 haveno perforations directly beneath the bottom of downcomers where risingbubbles would interfere with liquid flowing down the downcomers. Carbondioxide 9 is the dispersed phase. In operation tt rises through theextraction column because its density is lower than that of the coffeesolution. It forms small jets 10 which break into bubbles 11 in passingthrough the perforations 12 in the plates 6 and then through the layerof liquid maintained on the plates. A porous foam breaking material 13presenting exposed solid surface for breaking foam is spaced above theliquid layer. The porous material fills the available cross-section inthe extractor to prevent rising foam from passing through channelswithout contacting the material.

Suitable surface materials for foam breaking include stainless steel andpolytetrafluoroethylene. Stainless steel is preferable because it iswetted by the collapsed foam which then drains more easily by capillaryaction and gravity, returning to the solution layer. In contrast, thesurface of polytetrafluoroethylene tends to become loaded with collapsedfoam especially at high carbon dioxide flow rates. The effect is atendency to cause the column to "flood" and the aqueous solution toempty from the column.

Furthermore, polytetrafluoroethylene is difficult to fabricate with alarge surface area. A somewhat similar material, polyfluorinatedethylene-propylene, is much easier to fabricate. However, itdeteriorates mechanically by swelling in supercritical carbon dioxide.

A surface area of at least about 12 square centimeters per cubiccentimeter of knitted wire is needed to break foam. A large voidfraction minimizes resistance to flow of the carbon dioxide. Suitableporous material can be made from knitted stainless steel wire. Suchmaterials are sold by Koch Engineering Company, Inc., as well as others,for use as mist eliminators in distillation or evaporation equipment.Table I presents data on coffee foam breaking by knitted stainless steelwire materials in an extraction column under constant operatingconditions while decaffeinating a solution of green coffee solubles withsupercritical carbon dioxide.

                  TABLE I                                                         ______________________________________                                        Surface Area                                                                            3.6       11.2     14.8  20.0  23.0                                 cm.sup.2 /cm.sup.3                                                            Bulk Density                                                                            0.112     0.240    0.320 0.433 0.497                                g/cm.sup.3                                                                    % Voids   98.6      97.0     96.0  94.6  93.8                                 Foam-Breaking                                                                           Ineffective                                                                             Fair*    Good  Very  **                                   Effectiveness                      Good                                       ______________________________________                                         *Operable only at relatively very low carbon dioxide flow rate.               **Could not be evaluated because excessive pressure drop in the material      caused the column to flood.                                              

In operation foam 14 which forms from the carbon dioxide bubbles and thecoffee solution fills the space above the liquid and partiallypenetrates the foam-breaking material. Carbon dioxide 9 leaves the topof the foam-breaking material substantially free of liquid. Liquidformed by the collapse of the foam drains back to the liquid layer.

The term extraction stage is used to refer to a unit within which bothcontact and separation of the two fluids take place. A perforated plate6 is at the bottom of each stage. In operation of the extractionapparatus a very large area of contact is created by the carbon dioxidebubbles formed in passing up through the plate perforations, the gasflow rate being much greater than the liquid rate. This facilitatestransfer of caffeine to the carbon dioxide approaching an equilibriumconcentration.

Efficient extraction is effected by use of a number of extractionstages, stacked vertically, so that the fluids flow countercurrently,the stack comprising an "extraction column." The preferred number ofstages can be calculated by conventional means known to persons skilledin the art, such as the McCabe-Theile method. Although equilibriumdistribution of caffeine between the two fluids may be approached ineach stage, the countercurrent arrangement imposes a new equilibriumgoal in each successive stage so that the concentration of caffeine inthe coffee solution leaving the bottom of the extractor is far belowthat which would be in equilibrium with the carbon dioxide leaving thetop.

A major factor affecting the number of stages and the quantity of carbondioxide required is the "distribution coefficient." It is defined as theratio of the caffeine concentration in carbon dioxide to that in theaqueous solution after the two fluids have experienced sufficientcontact to reach equilibrium (when no further transfer of caffeine isdetected). Under the preferred operating conditions of this inventionthe distribution coefficient was found to average about 0.035, slightlylower at low caffeine concentrations and slightly higher at highconcentrations. Increasing the system pressure increased thedistribution coefficient. No effect of temperature on the distributioncoefficient was found within the preferred temperature range of 80°-85°C.

The system is designed and operated, as described below, so that a thicklayer of coffee solution is maintained on every plate. This assures thatthe gas jets issuing from the plate perforations can form droplets inthe liquid and provide the needed large contact area between the twofluids. If the layer is too thin, the gas will push away the liquidabove each plate perforation and minimize contact and extractionefficiency. Furthermore, sufficient liquid is needed to provide a liquidseal to prevent gas from rising through the downcomers instead ofthrough the plate perforations. The liquid height may be regarded as theintrastage interface level between liquid and gas. The depth determinesthe quantity of coffee extract inventory in the extractor. For practicalreasons it is desirable to minimize this inventory. Furthermore, the topof the liquid layer should be kept spaced below the bottom of the porousfoam-breaking material which becomes less effective if submerged inliquid. Experience has shown that the layer should be at least about 20mm and preferably about 35 to 50 mm deep.

Flow of the two fluids in opposite directions is driven by theirdifference in density. Both flows overcome inherent resistances whichproduce pressure drops as a function of flow rate. Any pressure dropincrease caused by an increased flow rate or increase in any flowresistance lowers the depth of liquid on the plates. The depth may beregarded an analogous to one leg of a manometer used to measure pressuredrop. Gas or foam which finds its way into the downcomers reduces theeffective liquid density and reduces the force available to drive theflow, also reducing the liquid layer depth.

Liquid pressure drop is caused by flow resistance in the downcomers andby flow direction changes in entering and leaving the downcomers. Gaspressure drop is affected by the number and size of perforations throughwhich the gas must pass and, to a small extent, by interfacial tensionwhich must be overcome to form bubbles of gas.

The porous foam-breaking material of this invention adds resistance tothe gas flow and would be expected to interfere with extractionoperation. It has been discovered, however, that in accordance with thisinvention foam-breaking performance is effective at velocities of about0.1 to 0.15 meters per second through the plate perforations. Theserates are typical of industrial sieve plate extractors operating withnon-foaming systems. The results shown in Table I were obtained at aperforation gas velocity of about 0.12 meters per second.

The foam-breaking material must be optimized for best results. Increasedexposed surface area increases foam-breaking action but also increasesthe gas pressure drop and depresses the liquid layer depth. Also,porosity (percent of voids) of the material affects pressure drop;materials of smaller porosities cause greater pressure drops.

In the practice of this invention, it was found advantageous to providedowncomers which have much larger cross-sectional area than inconventional design. Large downcomers reduce liquid velocity and permitdisengagement of foam which may be entrained into the downcomer alongwith liquid. The pressure drop can be regulated by installing a orificeat the bottom of each downcomer to reduce liquid depth on the plates.These orifices can also facilitate startup of the extractor by limitinginitial flow of gas up the downcomers.

The effectiveness of the foam-breaking surface also can affect liquidlayer depth. For example, use of a mesh with a relatively large voidpercentage and a relatively small surface area is expected to cause asmaller pressure drop. However, when there is insufficient area forefficient foam breaking, the foam tends to fill the voids and restrictfree flow of the gas with resulting excess pressure drop, reduced liquiddepth and, eventually, an extractor emptied of liquid.

Similarly, a bed of foam-breaking material with a height of 150 mmprovided more area and was more effective than a 75 mm bed of the samematerial despite expected increased pressure drop.

The viscosity of green coffee solution is very close to that of water.However, foam formed from the solution has a much higher viscosity thanthe solution and increases with increases in solution concentration.Very viscous foam fails to drain well from the porous material andincreases resistance to carbon dioxide flow through the porous material.A concentration not over about 27% solubles is preferred althoughsatisfactory operation has been carried out at somewhat higherconcentrations.

It has been found that the ability to eliminate the effects of foamingis critical to the operation of the decaffeination process. Failure todefoam well is manifested in the same way as excessive interstage orintrastage pressure drops. Liquid height on the trays declines, allowingfoam to enter the downcomers. The foam having a density intermediatebetween liquid and gas, there is no longer sufficient density differenceto drive the liquid downward by gravity. Liquid and foam may actuallyflow up the downcomers driven by the gas flow. Gas leaves the bottom ofthe column in place of liquid which has failed to come down. At the sametime, unless the liquid feed to the top of the column is stopped, liquidor foam may be carried out with the gas leaving the top and the columnis said to be "flooded".

During operation, the depth of liquid on the plates can be calculatedfrom the liquid inventory obtained by metering the coffee solutionentering and leaving the column since startup. Alternatively, it can bemeasured on shutdown by stopping both liquid and gas flows and drainingthe liquid. The liquid normally drains quickly if the foam breaking hasbeen operating well. If not, draining will be slow and it may benecessary to reduce system pressure to below about 70 bar in order todecrease the density of the gas sufficiently to allow foam to flowdownward and coalesce to a liquid.

In addition to controlling the intrastage interface level, control ofthe interface level above the top plate known as the principal interfacelevel is necessary. Conventionally in liquid-liquid columns the denserliquid is fed above the top plate and the rate of discharge of liquidfrom the bottom of the column is controlled by means of a throttlingvalve in the discharge line which responds to the top interface levelmeasured by an instrument. As the level rises incrementally thethrottling valve opens proportionally to provide control. This techniquewas found to be unstable in the present system because when the valveopened and liquid flow increased, the liquid pressure drop increased ineach stage causing the liquid depth on the plates to fall and resultingin a temporary rise in the top interface level. Furthermore, there was aconsiderable time delay until discharge rates could be sensed at thecolumn top. It was found that stable control was obtained by providing aconstant rate of decaffeinated liquid discharge from the bottom of thecolumn and arranging for variable feed rate of undecaffeinated liquidextract to the top of the column in response to the level instrument.

An alternative sieve plate column design commonly used for distillationoperations where there is a large density difference between liquid andvapor was found to be less suited for the present process. The liquidpath is not continuous and the principal interface is located below thebottom plate. That design has the advantage that intrastage interfacelevels are set by means of overflow weirs on each plate. However, liquidspilling over the weirs entrains gas to produce foam in the downcomerswhich prevents continued operation.

The extract decaffeination operation is but one part of a preferredprocess for decaffeinating coffee. Application of the decaffeinatedextract to decaffeinate green coffee beans and the recovery ofcaffeine-laden carbon dioxide are simultaneous operations which must beintegrated with the extract decaffeination. To take best advantage ofthe continuous extract decaffeination column, the other steps arepreferably continuous. Advantages of continuous operation includeproduct uniformity, simplified automatic control, minimal processinventory and generally smaller, more economical equipment.

Referring to FIG. 2, the preferred system comprises a solid-liquidextractor 15 in which both decaffeinated aqueous extract 16 andundecaffeinated coffee beans 17 move continuously in oppositedirections.

Although the countercurrent batch extraction system "water process"described in Berry et al, U.S. Pat. No. 2,309,092, could be used, it isnot a truly continuous extractor. The extent of decaffeination obtaineddepends, inter alia, on the bean residence time while caffeine isextracted by the green coffee solution. The system of the presentinvention has advantages over that of Berry. Berry et al required 8hours at 200° F. to reach 98% decaffeination. Table II gives resultsobtained with a continuous process at the same temperature.

                  TABLE II                                                        ______________________________________                                        Bean Residence Time                                                                            % Decaffeination                                             ______________________________________                                        5.3 hours        96.7                                                         5.8 hours        98.4                                                         6.4 hours        99.1                                                         ______________________________________                                    

The advantage of continuous extraction probably is that channeling instatic beds of beans is avoided.

The green bean extract travels in a continuous loop. Caffeine-laden("rich") extract 18 from the end of the bean extractor 15, where freshgreen beans 17 enter, is sent to the top of the extraction column 19where it meets carbon dioxide 20 which enters the bottom of the column.Decaffeinated ("lean") extract 16 from the bottom of the column isreturned to the opposite end of the bean extractor where thedecaffeinated beans 21 leave. Fresh water 22 must be added to make upfor moisture absorbed by the beans. This water is conveniently used towash extract from the surface of the decaffeinated beans before the wash23 goes to the bean extractor. Washing means 24 is needed to preventlumping of beans and loss of solids and flavor during subsequent drying.The dry decaffeinated beans 25 are discharged continuously from drier26.

In the preferred continuous process, caffeine must be removedsimultaneously from the carbon dioxide 27 which has decaffeinated thecoffee extract to allow re-use of the carbon dioxide for economy. Thismight be done by filtration through a bed of activated carbon but againeconomy demands recovery of the carbon. That is difficult because carbonholds caffeine tightly and carbon recovery may require incineration withassociated heating and cooling costs as well as loss of caffeine.

Removal of caffeine from the carbon dioxide by means of washing withwater is preferred. This can be accomplished in a second sieve plateextraction column 28 of similar design to the extractor 19 used todecaffeinate the extract. It was found that the same foam-breaking meansused in the extraction column is necessary for satisfactory operation ofthis carbon dioxide washing column. Pure water 29 is fed to the top ofthe column while the carbon dioxide containing dissolved caffeine 27 isfed to the bottom. Carbon dioxide 20 substantially free of caffeineexits the top of the column to be recycled to the bottom of the coffeeextract decaffeination column 19 to form a second loop. Water containingcaffeine 30 exits the bottom of the carbon dioxide washing column 28.Caffeine may be recovered from the water solution. The selectivity ofcarbon dioxide for caffeine is illustrated by the purity of the solidsobtained by drying the solution. It typically exceeds 95% caffeinebefore further refining.

It is desirable to operate both extraction columns in the carbon dioxideloop 19 and 28 at about the same pressure to minimize energy cost forcompression. Similarly it is best to operate both columns at about thesame temperature to minimize costs for heating and cooling. Waterdissolves in pressurized carbon dioxide to a small extent depending ontemperature. If the carbon dioxide washing column is hotter than thecoffee extract column, the circulating carbon dioxide will dilute theextract; if cooler, it will make the extract become more concentratedduring decaffeination.

To achieve a high degree of decaffeination of the coffee extract, thecarbon dioxide entering the extract decaffeination column must have nomore than about 2 to 3 parts per million of caffeine. This very lowlevel depends on the efficiency of the carbon dioxide washing column.Caffeine level in the carbon dioxide is not easily measured directly.However, a simple expedient was found to serve. A small portion of thelow-caffeine carbon dioxide stream is cooled and passed through a vesselwhere some water is allowed to condense and settle. Since the condensedwater is in equilibrium with the carbon dioxide, caffeine analysis of asample of this water may be used to calculate the caffeine level in thecarbon dioxide by applying a predetermined distribution coefficient ofcaffeine dissolved in carbon dioxide in contact with water. Thisdistribution coefficient is about 0.1. (The distribution coefficient forcarbon dioxide in contact with coffee extract was 0.035 which impliesthat coffee extract has a greater "affinity" for caffeine than doeswater.)

The effectiveness of the extract decaffeination column can be assessedby caffeine analyses of samples of the entering and exiting extractstreams, provided that the entering carbon dioxide is sufficiently lowin caffeine. Similarly, the effectiveness of the bean extractor can bemeasured by caffeine analyses of the raw green coffee and thedecaffeinated beans, provided that the caffeine level in thedecaffeinated extract is low.

Green coffee extract is a fertile medium for microbiological growth.Sanitary handling is essential to minimize contamination. In addition,the extract must be kept at elevated temperature of not less than about75° C. to prevent deterioration.

Fresh coffee extract carries a small amount of insoluble solidsincluding bits of chaff and particles of soil acquired by the beans onthe plantation or in handling and transport. To prevent accumulation ofthese solids in the extraction column, the extract may be centrifugedbefore being fed to the column. Some of these solids are readily removedin a low speed centrifuge with a centrifugal force of 1000 timesgravity, but a much better job is done with a higher speed unit at about10,000 times gravity. The solids removed represent less than 1% of thedry weight of the green beans and some of that weight, the chaff, wouldlater have been lost by burning during normal roasting. The removal ofsoil particles is believed to be responsible for an improved "cleaner"flavor in the decaffeinated roasted coffee as compared to theundecaffeinated coffee. A very small fraction of solid waxy materialapparently derived from a natural coating on the beans is alsopreferably removed by centrifugation. Unlike the other removable solids,the density of the waxy material is less than that of the extract.

The yield of decaffeinated green coffee is better than 95% of theundecaffeinated beans fed to the system. The difference representscaffeine extracted, chaff and insoluble solids removed in thecentrifuge, a trace amount of soluble material extracted with thecaffeine, and a very small amount of fines blown away while drying thedecaffeinated beans.

This invention provides the means for economically producing excellentquality decaffeinated coffee. Flavor and aroma present inundecaffeinated coffee is retained because processing time is short,conditions are gentle and contact with organic solvents is avoided. Ahigh degree of decaffeination is readily attained, exceeding 99% ifdesired. The process is economical because it is continuous, the beansare not handled at elevated pressure, industrial flow rates are employedin the extractor, and the outturn of decaffeinated coffee exceeds 95% ofthe weight of undecaffeinated coffee.

The preferred system is further described in the following example.

Example

A complete pilot plant is constructed and operated employing the processdisclosed. The principal equipment is arranged as shown in FIG. 2. Allthe equipment is constructed of 304 or 316 stainless steel or Inconel600.

The extraction column 19 for extraction of caffeine from green coffeeextract with carbon dioxide is a vertical cylinder 76 mm in diameter and12.8m tall containing 40 plates, spaced 254 mm apart. Each plate has 59perforations, 12 in FIG. 1, of 3.2 mm diameter arranged in triangularpitch on 6.4 mm centers. The total area of the perforations in eachplate is 10.5% of the column cross-sectional area. An open tube of 21 mminside diameter and 235 mm long is attached to each plate, serving asthe downcomer 8. Each plate is fitted with a TFE Teflon sheet lip-sealgasket in contact with the inside wall of the column. The distancebetween plates is maintained by four spacer tubes 6 mm in diameterlocated between adjacent plates. Rods 3 mm in diameter pass throughthese tubes as well as through holes in the plates and extend the lengthof the column.

Near the bottom of each downcomer an orifice plate is installedcontaining three perforations, each of 3 mm diameter. The foam-breakingmaterial is made of knitted stainless steel wire 0.108 mm in diameterand has a bulk density of 0.433 g/cm³ and a surface area of about 20square centimeters per cubic centimeter of mesh. About 250 g of thismaterial is fitted between successive plates starting 90 mm above eachplate and extending 150 mm upward. A portion of the mesh near the columnwall is extended downward toward the plate to insure effective drainageof liquid formed by breaking of foam.

The column is pressurized with carbon dioxide maintained at 414 ±1 barwith an automatically controlled makeup pump. A separate circulatingpump provides a flow of 2.80 kilograms per minute of carbon dioxide upthrough the column. This flow is equivalent to a velocity of 0.12 metersper second through the plate perforations. The carbon dioxide flow issensed by a differential pressure transmitter which measures pressuredrop across a section of the piping and controls an automatic throttlingvalve. The circulating pump also has an automatically regulated bypassvalve which maintains a constant pressure drop across the throttlingvalve.

The green coffee extract containing caffeine is pumped to the top of thecolumn. The speed of this pump is automatically controlled by adifferential pressure transmitter which senses extract level at the topof the column. The extract flow rate of 4.5 liters per hour ismaintained at the extract exit at the bottom of the column.

The column temperature of 82° C. is maintained by means of anautomatically controlled, steam heated exchanger through which thecarbon dioxide passes before entering the column. Also, fourautomatically controlled steam tracing lines are wrapped around thecolumn and covered with insulation.

A second sieve plate column of the same design, 28 in FIG. 2, is used towash the caffeine from the carbon dioxide. Deionized water is providedto the top of this second column by an automatic water pump and thewater, laden with caffeine, exits the bottom at a controlled rate of 30liters per hour. The temperature and pressure are close to thosemaintained in the coffee extract column.

Green coffee is fed at a constant rate of 3 kilograms per hour to oneend of the bean extractor, 15 in FIG. 2. At the same end, extract at25.4% solubles concentration and laden with caffeine exits. The extractis passed through a centrifuge to remove insoluble matter and then ispumped to the extract decaffeination column. The beans move through thebean extractor continuously with a residence time of about 6 hours andare maintained at 92° to 95° C. At the opposite end decaffeinatedextract from the extract column enters and decaffeinated beans leave.The decaffeinated beans are continuously elevated with an inclined screwconveyor fitted with spray nozzles supplied with deionized water to washoff the decaffeinated extract which coats the wet beans. The diluteextract formed by the water wash flows down the screw conveyor andenters the bean extractor. Washed beans are discharged from the upperend of the screw conveyor into the bean drier. A vibrating fluid beddrier serves to dry the beans. The beans are supported on a vibratingdeck provided with perforations to admit heated air from below fordrying. The deck is 2.75m long. A 75 mm deep bed of beans is maintainedin a fluidized state by the flow of drying air assisted by vibration ofthe deck. A bean temperature of about 95° C. is maintained near theentry point and the temperature is gradually reduced to about 80° C. asthe beans move through the drier. They are then cooled to about 45° C.The bean residence time is about 11/2 hours in the drier. The dischargedbeans have a moisture content of 8.8%.

It is necessary that the supply of extract provided by the beanextractor be matched to the demand for extract being fed to theextraction column. For this purpose a small tank is provided to hold asupply of extract before it is pumped to the column. A level sensor inthe tank is used to automatically adjust the quantity of wash water usedto wash the decaffeinated beans to maintain the required supply ofextract.

A small amount of carbon dioxide becomes dissolved into both thedecaffeinated extract and into the water carrying caffeine washed fromthe carbon dioxide. Most of this carbon dioxide can be recovered. Sincein this example it is not being recovered, the amount lost is made upwith carbon dioxide by the makeup pump described above.

In this example, the green coffee contains 1.09% caffeine. The drieddecaffeinated beans containing 0.015% caffeine, calculated on an equalmoisture basis, is 98.6% decaffeinated.

The discharge rate of decaffeinated beans under constant conditions fora period of three hours is at the rate of 2.895 kilograms per hour,corrected for the small difference in moisture between the decaffeinatedand undecaffeinated coffees. This rate of discharge is only 3.5% lessthan the bean feed rate.

After roasting, the flavor quality and strength of the decaffeinatedcoffee is excellent, equaling the fine quality of the green coffee beingfed to the process.

The invention is not limited to the precise details of structure shownand set forth in the preferred embodiments described, for obviousmodifications will occur to those skilled in the art to which theinvention pertains.

I claim:
 1. A process for decaffeination of green coffee comprising thesteps of:(a) extracting caffeine from green coffee beans by contactingthe beans with a decaffeinated aqueous solution of green coffee toprepare an extract containing caffeine and other soluble components ofgreen coffee and separating this extract from the green coffee beans nowsubstantially freed of caffeine, (b) feeding said extract to above theuppermost plate of a perforated plate extraction column having aplurality of plates, (c) feeding supercritical carbon dioxidecontinuously to below the lowest plate of the said column at a velocityat which, foam, formed by intermixing of the green coffee extract andthe supercritical carbon dioxide is broken by contact with a means forbreaking foam comprising a porous extended solid surface placed aboveeach perforated plate and extending across the column between successiveplates, (d) withdrawing said supercritical carbon dioxide together withdissolved caffeine continuously from the top of the column to removecaffeine from the green coffee extract, (e) removing the aqueous greencoffee extract, now substantially freed of caffeine, continuously fromthe bottom of the extraction column and recycling it to step (a) todecaffeinate additional green coffee beans, (f) maintaining the pressurewithin the said column at greater than about 330 bar, (g) maintainingthe temperature within said column in the range of about 80° to 100° C.,(h) maintaining the concentration of green coffee extract in the columnwithin the range of about 18 to 30 percent dissolved solids, (i)separating caffeine from the carbon dioxide exiting the top of thecolumn and recycling carbon dioxide, substantially freed of caffeine, tobelow the lowest plate of the column.
 2. The process for decaffeinationof green coffee of claim 1 wherein the foam breaking means of step (c)comprises a mesh means comprising a surface area greater than about 12square centimeters per cubic centimeter of mesh volume.
 3. The processfor decaffeination of green coffee of claim 1 wherein the foam breakingmeans of step (c) comprises a mesh means comprising knitted metal wirehaving a surface area greater than about 12 square centimeters per cubiccentimeter of mesh volume.
 4. The process for decaffeination of greencoffee of claim 1 wherein the foam breaking means of step (c) comprisesa mesh means comprising polytetrafluoroethylene having a surface areagreater than about 12 square centimeters per cubic centimeter of meshvolume.
 5. The process for decaffeination of green coffee of claim 1wherein the foam breaking means of step (c) comprises a mesh meanscomprising 300 series stainless steel wire having a surface area greaterthan about 12 square centimeters per cubic centimeter of mesh volume. 6.The process for decaffeination of green coffee of claim 1 wherein thefoam breaking means of step (c) comprises a mesh means comprising metalwire having a surface area of about 20 square centimeters per cubiccentimeter of mesh volume.
 7. The process for decaffeination of greencoffee of claim 1 wherein the foam breaking means of step (c) comprisesa mesh means comprising series 300 stainless steel wire having adiameter of about 0.10 millimeter, a mesh bulk density of about 0.43gram per cubic centimeter, a void percentage of about 95 percent, and asurface area of about 20 square centimeters per cubic centimeter of meshvolume.
 8. The process for decaffeination of green coffee of claim 1wherein the foam breaking means of step (c) comprises a mesh meanscomprising a surface area greater than about 12 square centimeters percubic centimeter of mesh volume and a vertical thickness of greater thanabout 75 millimeters.
 9. The process for decaffeination of green coffeeof claim 1 wherein the foam breaking means of step (c) comprises a meshmeans comprising a surface area greater than about 12 square centimetersper cubic centimeter of mesh volume and a vertical thickness of about150 millimeters.
 10. The process for decaffeination of green coffee ofclaim 1 wherein the foam breaking means of step (c) compirses a meshmeans comprising a surface area greater than about 12 square centimetersper cubic centimeter of mesh volume and a lower edge placed at or abovethe level of liquid on each of the perforated plates in the extractioncolumn.
 11. The process for decaffeination of green coffee of claim 1,2, 3, 4, 5, 6, 7, 8, 9 or 10 wherein the pressure in step (f) ismaintained at about 380 bar.
 12. The process for decaffeination of greencoffee of claim 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 wherein the pressure instep (f) is maintained at about 420 bar.
 13. The process fordecaffeination of green coffee of claims 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10wherein the temperature in step (g) is maintained in the range of about80° to 85° C.
 14. The process for decaffeination of green coffee ofclaim 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 wherein the concentration of theextract in step (h) is maintained in the range of about 22 to 27 percentdissolved solids.
 15. The process for decaffeination of green coffee ofclaims 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 wherein the concentration of theextract in step (h) is maintained in the range of about 26 percentdissolved solids.
 16. The process for decaffeination of green coffee ofclaims 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 wherein supercritical carbondioxide in step (c) is fed at a rate to maintain a velocity of about 0.1to 0.15 meters per second through the perforations in the plates in thecolumn.